1. Field of the Invention
The present invention relates to particulate matter detection sensors and particulate matter detection sensor units capable of detecting a quantity of particulate matter contained in target detection gas such as exhaust gas emitted from an internal combustion engine.
2. Description of the Related Art
In general, a diesel engine exhaust gas system is equipped with a diesel particulate filter (DPF). Such a DPF can eliminate conductive particulate matter PM from exhaust gas emitted from a diesel engine. That is, the DPF captures particulate matter PM contained in exhaust gas. The particulate matter PM captured in the DPF is oxidized at a high temperature of the exhaust gas. In other words, the DPF is regenerated at a high temperature. In order to execute the DPF regeneration process, it is necessary to calculate the quantity of particulate matter PM captured by the DPF. The DPF regeneration process is executed at the timing when the calculated quantity of particulate matter PM in the DPF exceeds a predetermined value.
There have been proposed various conventional methods of calculating the quantity of particulate matter PM captured in a DPF. One of the conventional methods uses a particulate matter PM detection sensor (or the PM detection sensor or smoke sensor for short) capable of detecting a quantity of particulate matter contained in exhaust gas. The detected quantity of particulate matter contained in such exhaust gas corresponds to the quantity of particulate matter accumulated in the DPF and the period of using the DPF. For example, Japanese patent laid open publication No. JP S60-1546 discloses such a PM detection sensor. In the conventional technique disclosed in Japanese patent laid open publication No. JP S60-1546, a pair of detection electrodes is formed on a substrate. The detection electrodes have a comb structure and face to each other on the substrate. The detection electrodes are electrically connected to detection terminals, respectively. The quantity of particulate matter PM contained in exhaust gas can be detected on the basis of a resistance change between the detection electrodes in the PM detection sensor. The resistance change occurs on the basis of the quantity of particulate matter PM accumulated between the detection electrodes.
In the PM detection sensor disclosed in Japanese patent laid open publication No. JP S60-4546, a pair of temperature adjusting electrodes is further formed on a front surface and a back surface of the substrate in addition to the detection electrodes. The temperature adjusting electrodes are electrically connected to a temperature adjusting terminal. This structure of the PM detection sensor makes it possible to correctly detect the concentration of particulate matter PM contained in exhaust gas without influence of the temperature dependence of the resistance between the detection electrodes.
Such a PM detection sensor is further applied to fault diagnosis of a DPF other than applied to the process of calculating the quantity of particulate matter PM accumulated in the DPF. That is, when a DPF failure occurs, exhaust gas containing an excess amount of particulate matter PM flows at the downstream of the DPF in the exhaust gas purifying system. In order to detect the occurrence of DPF failure, the PM detection sensor is placed at the downstream side of the DPF. The fault diagnosis is executed on the basis of the quantity of particulate matter PM contained in the exhaust gas at the downstream side of the DPF.
However, because being equipped with the temperature adjusting electrodes, the conventional particulate matter detection sensors, for example, disclosed in Japanese patent laid open publication No. JP S60-1546, suppress the influence of the temperature dependence of the detected resistance when the quantity of particulate matter PM contained in exhaust gas is detected, and therefore cannot detect the temperature of exhaust gas.
Further, the conventional PM detection sensor previously described has wires to be used for detecting the quantity of particulate matter PM and wires to be connected to the temperature adjusting electrodes. This causes a complicated circuit configuration and increased number of terminals.
Still further, in order to have the function of detecting the temperature of exhaust gas, it is necessary to have additional wires and terminals in the conventional particulate matter detection sensor. This increases the conventional problem previously described.